1. Field of the Invention
The present invention relates to an antiglare film to be provided on the surface of a window, display etc. In particular, it relates to an antiglare film to be provided on the surface of such displays as a liquid crystal display (LCD), CRT display, organic electroluminescence display (ELD), plasma display (PDP), surface-conduction electron-emitter display (SED) and field effect display (FED).
2. Description of the Related Art
Displays such as a liquid crystal display, CRT display, EL display and plasma display have some problems described below from the viewpoint of visibility.
External light reflects at looking and listening.
Surface glare (scintillation) occurs at the display surface by display light from the display.
Visibility is not good caused by dazzle of display light directly coming from the display without being diffused, etc.
Visibility is also degraded by such defect as unevenness of brightness.
In order to solve such lowering or degradation of visibility, it is known to arrange an antiglare film on the front face of a display.
As an antiglare film, for example, following techniques are known:
To arrange, on the surface of a display, an antiglare film having an antiglare layer having been subjected to embossing finish.
To arrange, on the surface of a display, an antiglare film having an antiglare layer on the surface of which is formed irregularity by mixing particles in a binder matrix.
In such antiglare layer, scattering phenomenon (surface diffusion) of light caused by surface irregularity is utilized.
Further, such antiglare layer is also known that, by mixing particles having a refraction index different from that of a binder matrix into the binder matrix, utilizes internal scattering (internal diffusion) of light based on the difference in refraction indices of the binder matrix and particles.
In an antiglare layer on the surface where irregularity is formed through embossing finish, the surface irregularity thereof can be completely controlled.
Consequently, reproducibility is good. However, when there is a defect or an adhered foreign substance on an emboss roll, endless defects occurs at the pitch of roll. Consequently, in the case of mass production, all the products have defect. Further, since the scattering only at the surface is utilized, there are following problems:
Abrasion resistance
Lowering of contrast
Occurrence of scintillation.
An antiglare film using a binder matrix and particles can be manufactured through a smaller number of processes than the antiglare film using embossing finish. Therefore, it can be manufactured inexpensively. Therefore, various embodiments of antiglare film are known (Patent Document 1).
In order to exert sufficient light diffusing performance, it is necessary to form a certain degree of surface irregularity on the surface of an antiglare film. However, there are following problems:
Lowering of contrast
Occurrence of scintillation caused by lens effect of the surface irregularity
Lowering of abrasion resistance
In addition, as methods for preventing the lowering of contrast and the occurrence of scintillation mentioned above, following techniques are known:
To improve transmission light and reflection light scattering performance by making irregularity figure of the surface large.
To improve transmission light by increasing the amount of particles to be added.
However, there is such a problem in the methods that the sharpness of a transmitted image is lowered.
As methods for improving the visibility etc. without lowering light scattering performance or the like, following techniques are known:
A technique in which binder matrix resin, spherical particles and amorphous particles are used in combination (Patent Document 2).
A technique in which binder matrix resin and plural particles having different particle sizes are used (Patent Document 3).
A technique including surface irregularity, wherein the cross-sectional area of the concave portion is defined (Patent Document 4).
In addition, in order to improve the visibility without lowering light scattering performance etc., there is also known such technique that uses scattering in an antiglare film and scattering at the surface of an antiglare film in combination.
The scattering within an antiglare film (internal diffusion) occurs by dispersing particles having a different refraction index from that of the binder matrix in a binder matrix such as resin.
The combined use of internal scattering and surface scattering leads to smaller surface irregularity compared with an antiglare film using surface scattering alone. Therefore, there are following advantages:
Improvement of contrast
Reduction of dazzling caused by lens effect of the surface irregularity
Improvement of abrasion resistance
For example, as the technique using internal scattering and surface scattering in combination, following techniques are known:
A technique wherein the internal haze (cloudiness) is 1-15%, and the surface haze (cloudiness) is 7-30% (Patent Documents 5, 6).
A technique wherein, while using binder resin and particles having the particle size of 0.5-5 μm, the difference in refraction indices of the resin and the particle is 0.02-0.2 (Patent Document 7).
A technique wherein, while using binder resin and particles having the particle size of 1-5 μm, the difference in refraction indices of the resin and the particle is 0.05-0.15. Further, techniques defining a solvent to be used, surface roughness etc. (Patent Documents 8, 9, 10, 11, 12).
A technique wherein, using binder resin and plural types of particles, the difference in refraction indices of the resin and the particle is 0.03-0.2 (Patent Documents 13, 14).
There are also known following techniques that reduce lowering of contrast, hue variation etc. when a viewing angle is altered: In the technique, the surface haze (cloudiness) is 3 or more. Further, the difference between the haze value in the direction of normal line and the haze value in the direction of ±60° is 4 or less (Patent Documents 15, 16, 17, 18). A technique wherein center line average roughness (Ra) is 0.2 μm or less is also known (Patent Document 19). A technique wherein the center line average roughness (Ra) is 0.02-1 μm, and the ten point average roughness (Rz)/Ra is 30 or less is also known (Patent Document 20, 21).
Since an antiglare film is mainly arranged on the front face of a display, abrasion resistance is required. In order to improve the abrasion resistance, it is necessary to improve the hardness of an antiglare film. Therefore, there is known such technique as using an ionizing radiation-curing resin binder, silica particles and silicone particles in order to manufacture an antiglare film having a high hardness without lowering the display image quality of a display (Patent Document 21).
In an antiglare film having particles and a binder matrix on a transparent base material, it is desirable that an average particle size of a particle is smaller than an average film thickness of an antiglare layer. The reason is, in the case where an average particle size of a particle is equal to or more than an average film thickness of an antiglare layer, optical property of an antiglare layer changes mercurially for a film thickness fluctuation of an antiglare layer. In the case where an average particle size of a particle is equal to or more than an average film thickness of the antiglare layer, even if the film thickness fluctuation occurring in machinery coating is equal to or less than ±5%, the difference is easily recognized visually, and nonuniformity in appearance appears notably. Therefore, when an antiglare film is manufactured, probability of occurrence of defective appearance becomes high, and there is a problem such that deterioration in the productivity can be easily caused.
On the other hand, in the case where an antiglare layer is formed by using a particle whose average particle size is smaller than an average film thickness of an antiglare layer, control of dispersion property of a particle becomes important. In the antiglare layer, particles often aggregate without dispersing, and when a particle aggregate, the convex portion having a big difference of height is formed on the surface of an antiglare layer, therefore the external light reflection preventive property (antiglare property) can be obtained, however, there is a problem such that the image sharpness property is drastically lowered.
In particular, in the case where an organic particle comprising an organic material is used as a particle, specific gravity of an organic particle is often equal to or less than a binder matrix, an organic particle is hard to be buried into an antiglare layer, and the above-mentioned problem becomes notable.
Therefore, in the present invention, one purpose is to provide an antiglare film having a high image sharpness property in an antiglare layer including a particle whose average particle size is less than an average film thickness of an antiglare layer.
[Patent Document 1] U.S. Pat. No. 5,387,463
[Patent Document 2] JP-A-2003-260748
[Patent Document 3] JP-A-2004-004777
[Patent Document 4] JP-A-2003-004903
[Patent Document 5] Japanese Patent No. 3507719
[Patent Document 6] U.S. Pat. No. 6,343,865
[Patent Document 7] JP-A-11-326608
[Patent Document 8] Japanese Patent No. 3515426
[Patent Document 9] U.S. Pat. No. 6,696,140
[Patent Document 10] U.S. Pat. No. 7,033,638
[Patent Document 11] US Patent Published Application No. 2002-0150722
[Patent Document 12] US Patent Published Application No. 2004-0150874
[Patent Document 13] Japanese Patent No. 3515401
[Patent Document 14] U.S. Pat. No. 6,217,176
[Patent Document 15] JP-A-11-160505
[Patent Document 16] U.S. Pat. No. 6,111,699
[Patent Document 17] U.S. Pat. No. 6,327,088
[Patent Document 18] U.S. Pat. No. 6,480,249
[Patent Document 19] JP-A-2003-149413
[Patent Document 20] JP-A-2004-125958
[Patent Document 21] JP-A-2004-082613
[Patent Document 22] US Patent Published Application No. 2004-0071986